1. Technical Field of the Invention
The present invention relates to aqueous dispersions of polyurethane resins, aqueous dispersions of polyurethane resin graft polymers for which the former are both starting materials and stabilizers, and water-based paint compositions.
2. Prior Art
Water-based paint compositions have come to be widely used to replace solvent-based paints because they are superior from the point of view of environmental preservation and worker safety. For example, in the field of base coats for painting the outer skin of automobiles, etc., there has been a progressive change from solvent-based base coats using large quantities of organic solvents to water-based base coats using water as a carrier.
In Japanese Unexamined Patent 2-64116 (EP 353,797 A1) a polyurethane/acrylic emulsion varnish made by mixing an ethylenic unsaturated monomer and a polyurethane resin and emulsion polymerizing in an aqueous medium is disclosed as a resin in aqueous dispersion which can be employed in water-based base coats. However, during the synthesis of this polyurethane/acrylic emulsion varnish it is difficult to form a stable emulsion of the ethylenic unsaturated monomer in water by means of the surfactant capacity of the urethane oligomer, and consequently a low-molecular-weight emulsifier has to be employed to form a the ethylenic unsaturated monomer into a stable emulsion. This narrows the scope for selection of ethylenic unsaturated monomers, and narrows the scope for designing resins that can be synthesized. There is also the problem that the water-resistance of paint films in which these emulsions are employed is poor. Moreover, the ethylenic unsaturated monomer is employed as a solvent in order to lower the viscosity of the urethane oligomer, and therefore a polymerization inhibitor is necessary in order to prevent thermopolymerization of the ethylenic monomer during the synthesis; as a consequence percentage polymerization is lowered and it is difficult to obtain the intended varnish when the varnish emulsion is emulsion polymerized using urethane as the emulsifier.
A water-based coating composition disclosed in Japanese Unexamined Patent 3-174479 which comprises an acrylic polymer of specific composition and a polyurethane dispersion is also basically a mixed dispersion of an acrylic polymer and polyurethane, which has the problem of poor stability.
Japanese Unexamined Patent 61-2720 also discloses an acrylic emulsion made by emulsion polymerizing ethylenic unsaturated monomer(s) using an anionic or cationic water-dispersible urethane oligomer as the emulsifying agent. However, in synthesizing this varnish emulsion it is difficult to form the ethylenic unsaturated monomer into a stable emulsion in the aqueous medium by means of the surfactant capacity of the water-dispersible urethane oligomer alone, which narrows the scope for selection of ethylenic unsaturated monomers and narrows the scope for designing the resins that can be synthesized. In addition, the water resistance of the resulting paint films is poor because of the surfactant used during synthesis. Moreover, there is the problem that when employed in mixtures with other starting materials storage stability is poor and coagulation is prone to occur.
Japanese Examined Patent 7-2827 also discloses water-based paint compositions which contain an acrylic emulsion made by polymerizing ethylenic unsaturated monomers using as an emulsifying agent a macromolecular composition having urethane bonds and amphoteric ionic groups. However, the synthesis of the macromolecular composition which acts as an emulsifying agent is performed by extending the chain of a urethane oligomer using a primary and/or secondary polyamine as a chain extension agent to obtain a macromolecular composition, and consequently there is the problem that paint films containing this macromolecular composition have poor basic paint film properties such as resistance to heat-induced yellowing and resistance to light-induced yellowing, etc., as a result of the chain extension agents above.
The purpose of the present invention is to offer aqueous dispersions of polyurethane resins which have outstanding dispersion stability and which can polymerize with various ethylenic unsaturated monomers, which are aqueous dispersions formed by dispersing in an aqueous medium a polyurethane resin which has a molecule which includes free-radical-polymerizable unsaturated bonds.
Another purpose of the present invention is to offer aqueous dispersions of polyurethane resin graft polymers in which the quantity of coagulation during polymerization is small, which have outstanding dispersion stability, and can also form paint films with outstanding appearance and water resistance, etc., which are aqueous dispersions of a polyurethane graft copolymer obtained by polymerizing an aqueous dispersion of a polyurethane resin above with ethylenic unsaturated monomer(s).
A separate purpose of the present invention is to offer water-based paint compositions which have outstanding storage stability and can also form paint films with outstanding appearance and basic paint film performance traits such as water resistance, etc., which are water-based paint compositions which include an aqueous dispersion of a polyurethane resin graft polymer above.